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30 Cards in this Set
- Front
- Back
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medium access control (MAC) |
Medium access control comprises all mechanisms that regulate user access to a medium using SDM, TDM, FDM, or CDM. MAC is thus similar to traffic regulations in the highway/multiplexing example introduced in chapter 2. |
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carrier sense multiple access with collision detection, (CSMA/CD) |
A sender senses the medium (a wire or coaxial cable) to see if it is free. If the medium is busy, the sender waits until it is free. If the medium is free, the sender starts transmitting data and continues to listen into the medium. If the sender detects a collision while sending, it stops at once and sends a jamming signal. |
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Why does CSMA/CD fail in wireless networks? |
CSMA/CD is not really interested in collisions at the sender, but rather in those at the receiver. The signal should reach the receiver without collisions. But the sender is the one detecting collisions. This is not a problem using a wire, as more or less the same signal strength can be assumed all over the wire if the length of the wire stays within certain often standardized limits. If a collision occurs somewhere in the wire, everybody will notice it. It does not matter if a sender listens into the medium to detect a collision at its own location while in reality is waiting to detect a possible collision at the receiver. |
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Hidden and exposed terminals |
The transmission range of A reaches B, but not C (the detection range does not reach C either). The transmission range of C reaches B, but not A. Finally, the transmission range of B reaches A and C, i.e., A cannot detect C and vice versa.
C also starts sending causing a collision at B. But A cannot detect this collision at B and continues with its transmission. A is hidden for C and vice versa. |
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Near and far terminals |
Consider the situation as shown in Figure 3.2. A and B are both sending with the same transmission power. As the signal strength decreases proportionally to the square of the distance, B’s signal drowns out A’s signal. As a result, C cannot receive A’s transmission. |
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Space Division Multiple Access (SDMA) |
is used for allocating a separated space to users in wireless networks.
The mobile phone may receive several base stations with different quality. A MAC algorithm could now decide which base station is best, taking into account which frequencies (FDM), time slots (TDM) or code (CDM) are still available (depending on the technology). Typically, SDMA is never used in isolation but always in combination with one or more other schemes. The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division mul- tiplexing (SDM) (see section 2.5.1).
Single users are separated in space by individual beams. |
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Frequency division multiple access (FDMA) |
comprises all algorithms allocat- ing frequencies to transmission channels according to the frequency division multiplexing (FDM) scheme
Allocation can either be fixed (as for radio stations or the general planning and regulation of frequen- cies) or dynamic (i.e., demand driven).
Channels can be assigned to the same frequency at all times, i.e., pure FDMA, or change frequencies according to a certain pattern, i.e., FDMA combined with TDMA. The latter example is the common practice for many wireless systems to circumvent narrowband interference at certain frequencies, known as frequency hopping. |
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frequency division duplex (FDD) |
The two directions, mobile station to base sta- tion and vice versa are now separated using different frequencies.
Both parties have to know the frequencies in advance; they cannot just listen into the medium. The two frequencies are also known as uplink, i.e., from mobile station to base sta- tion or from ground control to satellite, and as downlink, i.e., from base station to mobile station or from satellite to ground control. |
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time division multiple access (TDMA) |
Using only one frequency - made up of many channels separated in time at the same frequency
Almost all MAC schemes for wired networks work according to this principle, e.g., Ethernet, Token Ring, ATM etc. (
Synchronization between sender and receiver has to be achieved in the time domain. |
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Fixed TDM |
The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern. This results in a fixed bandwidth and is the typical solution for wireless phone systems.
If synchronization is assured, each mobile station knows its turn and no interference will happen. The fixed pattern can be assigned by the base station, where competition between different mobile stations that want to access the medium is solved. |
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Fixed access patterns |
guarantee a fixed delay – one can transmit, e.g., every 10 ms as this is the case for standard DECT systems. TDMA schemes with fixed access patterns are used for many digi- tal mobile phone systems like IS-54, IS-136, GSM, DECT, PHS, and PACS.
fit perfectly well for connections with a fixed bandwidth. |
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time division duplex (TDD) |
Assigning different slots for uplink and downlink using the same frequency
Uplink and downlink are separated in time. Up to 12 different mobile stations can use the same frequency without interference using this scheme. Each connection is allotted its own up- and downlink pair. |
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Classical Aloha |
a scheme which was invented at the University of Hawaii and was used in the ALOHANET for wireless connection of several stations.
Aloha neither coordinates medium access nor does it resolve contention on the MAC layer.
Instead, each station can access the medium at any time. This is a random access scheme, without a central arbiter controlling access and without coordination among the stations. If two or more stations access the medium at the same time, a collision occurs and the transmitted data is destroyed. Resolving this problem is left to higher layers (e.g., retransmission of data). |
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Slotted Aloha |
The first refinement of the classical Aloha scheme is provided by the introduction of time slots (slotted Aloha). In this case, all senders have to be synchronized, transmission can only start at the beginning of a time slot
access is not coordinated.
the introduction of slots raises the throughput from 18 per cent to 36 per cent, i.e., slotting doubles the throughput.
but they cannot give any hard transmission guarantees, such as maximum delay before accessing the medium, or minimum throughput. |
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carrier sense multiple access (CSMA) |
sensing the carrier before accessing the medium.
however, hidden terminals cannot be detected |
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non-persistent CSMA |
stations sense the carrier and start sending immediately if the medium is idle. If the medium is busy, the station pauses a random amount of time before sensing the medium again and repeating this pattern. |
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p-persistent CSMA |
systems nodes also sense the medium, but only transmit with a probability of p, with the station deferring to the next slot with the probability 1-p, i.e., access is slotted in addition. |
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1-persistent CSMA systems |
all stations wishing to transmit access the medium at the same time, as soon as it becomes idle. This will cause many collisions if many sta- tions wish to send and block each other. To create some fairness for stations waiting for a longer time, back-off algorithms can be introduced, which are sensi- tive to waiting time as this is done for standard Ethernet |
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CSMA/CA |
CSMA with collision avoidance
Here sensing the carrier is combined with a back-off scheme in case of a busy medium to achieve some fairness among competing stations. |
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(EY-NMPA) |
elimination yield – non-preemptive multiple access
used in the HIPERLAN 1 specification. Here several phases of sensing the medium and accessing the medium for contention resolution are interleaved before one “winner” can finally access the medium for data transmission. Here, priority schemes can be included to assure preference of certain stations with more important data. |
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reservation mechanisms |
A general improvement of Aloha access systems can also be achieved by reservation mechanisms and combinations with some (fixed) TDM patterns. These schemes typically have a reservation period followed by a transmission period. During the reservation period, stations can reserve future slots in the transmission period. |
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(DAMA) |
demand assigned multiple access
a scheme typical for satellite systems. DAMA, as shown in Figure 3.7 has two modes. During a contention phase following the slotted Aloha scheme, all stations can try to reserve future slots.
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(PRMA) |
packet reservation multiple access
Here, slots can be reserved implicitly according to the following scheme. A certain number of slots forms a frame (Figure 3.8 shows eight slots in a frame). The frame is repeated in time (forming frames one to five in the example), i.e., a fixed TDM pattern is applied.
A base station, which could be a satellite, now broadcasts the status of each slot (as shown on the left side of the figure) to all mobile stations. All stations receiving this vector will then know which slot is occupied and which slot is currently free.
As soon as a station has succeeded with a reservation, all future slots are implic- itly reserved for this station. This ensures transmission with a guaranteed data rate.
The slotted aloha scheme is used for idle slots only, data transmission is not destroyed by collision. |
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reservation TDMA |
Each station is allotted its own mini-slot and can use it to reserve up to k data-slots. This guarantees each station a certain bandwidth and a fixed delay. Other stations can now send data in unused data-slots as shown. Using these free slots can be based on a simple round-robin scheme or can be uncoordinated using an Aloha scheme. |
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(MACA) |
Multiple access with collision avoidance
presents a simple scheme that solves the hidden terminal problem, does not need a base station, and is still a random access Aloha scheme – but with dynamic reservation.
With MACA, A does not start its transmission at once, but sends a request to send (RTS) first. B receives the RTS that contains the name of sender and receiver, as well as the length of the future transmission. This RTS is not heard by C, but triggers an acknowledgement from B, called clear to send (CTS). The CTS again contains the names of sender (A) and receiver (B) of the user data, and the length of the future transmission. This CTS is now heard by C and the medium for future use by A is now reserved for the duration of the transmis- sion. After receiving a CTS, C is not allowed to send anything for the duration indicated in the CTS toward B. |
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polling |
Where one station is to be heard by all others, polling schemes may be applied
Polling is a strictly centralized scheme with one master station and several slave stations. The master can poll the slaves according to many schemes: round robin (only efficient if traffic pat- terns are similar over all stations), randomly, according to reservations (the classroom example with polite students) etc. The master could also establish a list of stations wishing to transmit during a contention phase. After this phase, the station polls each station on the list. |
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(ISMA) |
inhibit sense multiple access
This scheme, which is used for the packet data transmission service Cellular Digital Packet Data (CDPD) in the AMPS mobile phone system, is also known as digital sense multiple access (DSMA). Here, the base station only signals a busy medium via a busy tone (called BUSY/IDLE indicator) on the downlink (see Figure 3.13). After the busy tone stops, accessing the uplink is not coordinated any further. |
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(CDM) |
code division multiplexing
Code division multiple access (CDMA) systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference. The main problem is how to find “good” codes and how to separate the signal from noise generated by other signals and the environment.
A code for a certain user should have a good autocorrelation and should be orthogonal to other codes. Orthogonal in code space has the same meaning as in standard space |
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autocorrelation |
The Barker code (+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1), for example, has a good autocorrelation, i.e., the inner product with itself is large, the result is 11. |
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(SAMA) |
spread Aloha multiple access
is a combination of CDMA and TDMA
SAMA works as follows: each sender uses the same spreading code (in the example shown in Figure 3.19 this is the code 110101).4 The standard case for Aloha access is shown in the upper part of the figure. Sender A and sender B access the medium at the same time in their narrowband spectrum, so that all three bits shown cause a collision. |
